It has become increasingly desirable to reduce the size of power-producing or thrust-producing devices such as gas turbine engines. Gas turbine engines typically include one or more shafts that include compressors, bypass fans, and turbines. Typically, air is forced into the engine and passed into a compressor. The compressed air is passed to a combustor, and at high temperature and pressure the combustion products are passed into a turbine. The turbine provides power to the shaft, which in turn provides the power to the compressor and bypass fan or gearbox. Thrust is thereby produced from the air that passes from the bypass fan, as well as from the expended in the turbine combustion products.
However, air can be thermodynamically inefficient, especially during cruise operation of the engine (such as in an aircraft). Air that enters the engine is of low pressure, therefore low density. In order to reach the needed pressure and temperature at the combustor exit, the air is compressed to very high pressure ratios and heated up to very high temperatures in the combustors. In order to provide adequate mass flow rate, significant volume flow rate of the low density air is pumped through high pressure ratio consuming significant amount of power. As a result, the engines are made of large and heavy components, consume large amounts of fuel, and may include significant operational and maintenance expenses to cope with high combustion temperatures.
Some gas turbine engines include multiple stages and shafts to further improve thermodynamic efficiency. That is, some systems may include various compression stages that increase the pressure in each stage, providing very high pressure ratios that is passed to combustion, and expansion of the combustion products may also be through multiple stages, as well. For instance, a gas turbine may have Low Pressure (LP) and High Pressure (HP) shafts that correspond with respective LP and HP compressors and turbines, further improving the thermodynamic efficiency over a single stage engine. Or, such systems may include multiple compression and expansion stages.
One traditional option includes using an intercooler, to further improve thermodynamic efficiency by cooling the compressed air between compression stages (i.e., between LP and HP compressors). In fact, many gas turbine engines have been designed and implemented to improve thermodynamic efficiency.
However, these traditional gas turbine engines use air as an incoming working fluid that is used in the combustion process. As such, although traditional gas engines have made great strides in improving thermodynamic efficiency, such systems face a fundamental challenge of low density incoming air that is compressed in very high pressure ratios and heated up to very high temperatures. This fundamental challenge results in gas turbine engines that are generally quite large, to accommodate the large compression ratios. Such large components result in overall aircraft efficiencies because of the large amounts of mass that are used to build the engines, to meet the material needs for such large pressure ratios and high combustion temperatures.
To reduce component size and complexity, some power-producing or thrust-producing devices include a super-critical carbon dioxide (s-CO2) system that provides significantly improved efficiencies compared to a Brayton cycle and other air-based systems by operating in a super-critical region (operating at a temperature and pressure that exceed the critical point). Close to the critical point and in the super-critical region, the combination of thermodynamic and transport properties of CO2 provide a tremendous opportunity for high power energy extraction and in a small footprint relative to, for instance, an air-based thermodynamic system (such as a Brayton cycle).
Gas turbine engines have high reliability requirements. If a leak occurs in a gas turbine engine driven by supercritical CO2 cycle and the CO2 charge is lost, the engine performance degradation may reach a critical level. Thus, it may be beneficial to section heat exchangers of the engine in order to provide ability to switch off leaking sections and improve the engine reliability.
As such, there is a need to improve reliability, thermodynamic efficiency, and reduce overall size and mass in gas turbine engines.